If it were up to me, it would be co-op squad-based missions. Regardless of how ME2 ended (assuming Shep surviving is canonical) the ME universe is gearing up for a war. So it would fit to have the game place you in the shoes of a recruit and get sent on whatever objective based missions with your friends/fellow recruits in a side-story that is parallel, but not necessarily directly related to Shep's story.

I would like this the best. 2 or 4 player co-op with either a short campaign or individual missions where nobody plays as Shepard.